What's the best interface piece to start with?

[Delameko Stone]

Hi,

I decided to give UI design a go and thought I'd start with something easy like the player window. But I'm completely confused. I can't find the images for it (I thought I could just re-colour them or something) but all I succeeded in doing was making the bar completely vanish.

So what I'm actually asking, is what is the best/easiest UI piece to edit/alter, so I can learn the ropes..?

Thanks very much,